Flexible polyurethane foams are well recognized articles of commerce. The two most common classifications of flexible polyurethane foams are conventional and high resilience (HR). Flexible foams may also be characterized by the process used in production, either molded or free rise. Free rise foams are often made in a continuous slabstock process. In slabstock foam, the reactive foam forming ingredients, including any necessary catalysts, blowing agents, and foam stabilizing surfactants, are mixed and deposited on a moving, and generally lined, conveyor belt where the foam is allowed to rise freely. After cure, the foam is then sliced to the appropriate thickness for its intended use, for example as seat cushions, mattresses, carpet underlay, and the like. Molded foams are typically manufactured. within an enclosed chamber having the shape of the desired finished article. HR foam is manufactured by both free rise and molded processes.
High Resilience (HR) foam is defined by ASTM Method D3770-91, although the industry generally recognizes a wider category of foams which may be designated as HR foams. In particular, foams manufactured with HR chemicals but having densities lower than the limits specified in D3770-91 are also included in the wider category. These lower density foams are also included in the HR designation employed in this application. In general, HR foams are characterized by higher comfort or support factor and higher resilience than non-HR foam or "conventional" foam. HR foam is generally prepared by employing as the isocyanate-reactive component, a polyoxyalkylene polyol containing a stably dispersed polymer phase, a low molecular weight crosslinker/extender, generally diethanolamine, water as a reactive blowing agent, and amine and/or tin catalysts. While molded HR foam often employs methylene diphenylene diisocyanate or polymethylene polyphenylene polyisocyanate, often in specific isomer ratios and often in combination with toluene diisocyanate (TDI), HR slab-stock foam is generally prepared solely or predominately with TDI, either as an 80/20 or 65/35 blend of the 2,4-, and 2,6-isomers.
The surfactants used for conventional slab-stock foam and HR foam also differ, the latter generally employing a less potent surfactant such as the silicone surfactants described in U.S. Pat. No. 4,690,955. These low potency silicones are characterized by their lower molecular weight and minimal number of siloxane moieties in their molecular backbone, typically 20 or less. In some cases, non-silicone low potency surfactants may be used successfully.
The polyol polymer dispersions used in HR foam contain stably dispersed polymer particles generally prepared by in situ polymerization of polymerizable monomers in a polyoxyalkylene base polyol. One type of polyol polymer dispersion is the product obtained by the in situ polymerization of one or more vinyl monomers such as styrene and acrylonitrile in a polyoxyalkylene base polyol. Such polyol polymer dispersions are termed "polymer polyols" herein. A further polyol polymer dispersion is the type prepared by the in situ polymerization reaction between a di- or polyisocyanate and a low molecular weight isocyanate reactive species such as water, hydrazine, diamines, or alkanolamines. Such polyol polymer dispersions are termed "polymer-modified polyols" herein. While polyol polymer dispersions of either type may be made with the solids level required for a particular HR foam, i.e. in the range of 2 to 35 or more weight percent solids, it is common to prepare the polyol polymer dispersion at the highest solids level practical and then dilute the polyol polymer dispersion with additional polyol, which may be the same or different from the polyol polymer dispersion base polyol, to obtain the desired solids level. In this manner, maximum use is made of polyol polymer dispersion production capacity. In the past, it has been found, in general, that the polyoxyalkylene base polyol and any additional polyol subsequently blended to prepare the polyol component must have a high primary hydroxyl content in order to provide suitable reactivity. This is typically achieved by reacting ethylene oxide onto a polyoxypropylene core.
The high water levels used in preparing HR foams, particularly in the lower density range, create a problem with regard to processing latitude. In particular, it has proven difficult to process foams at isocyanate indexes of less than 100 and greater than 115. In U.S. Pat. No. 5,171,759, processing latitude is increased by inclusion in the conventionally catalyzed polyol component, a first, higher functionality polyol containing between 8 and 25 weight percent oxyethylene moieties, and a second polyol containing 70 weight percent or more polyoxyethylene moieties.
In U.S. Pat. No. 5,010,117, the use of polyoxypropylene polyols having a low monol content reflected by an unsaturation of less than 0.040 meq/g, as measured by ASTM D-2849-69 "Testing Urethane Foam Polyol Raw Materials", is suggested as a means of improving foam green strength and compression set. However, the patent exemplifies only molded foam prepared from high primary hydroxyl content polyols, and fails to examine polyoxypropylene polyols having unsaturations of less than 0.027 meq/g. Also, no examples are cited in which a double metal cyanide catalyst was used to prepare such polyols as described below.
Double metal cyanide (DMC) catalysts were discovered in the decade of the 1960's to be efficient oxyalkylation catalysts suitable for preparing polyoxyalkylene polyether polyols having notably lower levels of unsaturation, and thus monol content, than polyols prepared by traditional base catalysis. Unsaturations in the range of 0.018 to 0.025 meq/g were achieved. However, the cost/activity ratio of such catalysts coupled with the difficulty of removing catalyst residues from the polyol product prevented commercialization. Improved catalysts such as those disclosed in U.S. Pat. No. 5,158,922 showed higher activity and lowered unsaturation further, to the range of 0.015 to 0.018 meq/g. However, again, cost and processing difficulties prevented commercialization.
The use of double metal cyanide catalyzed polyols in HR foam production has not been actively pursued due to difficulties in achieving high primary hydroxyl levels through capping with ethylene oxide. DMC catalysts tend to homopolymerize ethylene oxide rather than add to existing secondary hydroxyl moieties, thus restricting the primary hydroxyl levels to less than 50% under normal production conditions. Introducing strongly basic catalysts for the ethylene oxide addition stage adds significant additional cost and complexity to the process.
Most recently, the ARCO Chemical Company has developed new double metal cyanide complex catalysts which offer exceptionally high catalytic activity coupled with the ability to remove catalyst residues by simple filtration. Polyoxyalkylene polyols prepared with such catalysts have exceptionally low levels of unsaturation, in the range of 0.003 to 0.010 meq/g.
The higher molecular weights and functionalities of DMC catalyzed polyols and lack of monofunctional species which serve as chain terminators in polyurethane polymerization reactions has led to the belief that use of such polyols to prepare polyurethanes will lead to improved properties and superior performance in many applications. However, it has been found that ultra low monol, ultra low unsaturation polyols produced with DMC catalysts are not simply "drop-in" replacements for conventional base catalyzed polyols. For example, R. L. Mascioli, "Urethane Applications for Novel High Molecular Weight Polyols," 32ND ANNUAL POLYURETHANE TECHNICAL/MARKETING CONFERENCE, Oct. 1-4, 1989, disclosed that polyurethane flexible foams prepared from a c.a. 11,000 Da molecular weight triol rather than a conventionally catalyzed 6200 Da triol produced a stiff and boardy foam. Due to the higher molecular weight of the polyol, a softer foam would have been expected.
Moreover, it has been found that addition of as little as 20 weight percent of a polyol, produced at least in part by DMC catalyzed oxypropylation, to the polyol component of an HR foam formulation results in a commercially unacceptable "tight" foam which exhibits severe shrinkage. Increasing the primary hydroxyl level by capping with a mixture of EO and PO in the latter stages of the DMC catalyzed polymerization did not avoid the tightness problem. Moreover, even higher primary hydroxyl, low unsaturation polyols prepared by DMC catalyzed oxypropylation followed by base catalyzed capping with ethylene oxide failed to solve the shrinkage problem.
Double metal cyanide complex catalysts capable of preparing very low unsaturation polyols offer the potential for preparing polyurethanes with improved physical properties. Future development may also result in the opportunity to produce polyoxyalkylene polyols at lower prices. However, in order to make use of these potential advantages in HR polyurethane foam, it is necessary to eliminate the shrinkage and tightness of foams prepared from these polyols. It is also preferable to produce such foams from polyols having moderate primary hydroxyl levels that can be achieved with DMC catalyst without resorting to base catalyzed addition of ethylene oxide at the end of the polymerization.